i'm having the same problem. i did a `grep -R portage /etc/*` to try to find an entry somewhere, but couldn't find anything. i'm not sure if OS X uses /etc/passwd anymore (there's an /etc/master.passwd as well).

i can't take credit for this, i found it on discussions.info.apple.com but i tweaked what they said to be a little more reasonable.

Run /Applications/Utilities/NetInfo Manager. Go to users in the first list box. Find portage. in the directory menu, choose New Property (or use <Command>+<Shift>+N) and add the properties shell with the value /bin/false and home with the value /dev/null

i used the postfix user as a template and added passwd, realname, etc as well. the guys at discussions.info.apple.com suggested just deleting the whole user entry, but i'd like to think its there for a reason.